1. Technical Field
The present invention relates to injector drive devices that supply valve-opening voltage to injectors of cylinders in a multi-cylinder internal combustion engine, and more particularly relates to an injector drive device with improved freedom in setting injection timing when different cylinders have adjacent injection timings.
2. Related Art
A drive device for a gasoline direct injector that directly injects pressurized gasoline into a combustion chamber (into a cylinder) is provided with a high-voltage power source to boost voltage from a battery, so that an injection valve can be opened in a stable manner even under higher fuel pressure. Such injector drive devices perform control where high voltage is temporarily applied when starting to open the valve, the current is increased to open the valve with an electromagnet, and thereafter the voltage is lowered and the opened valve state is maintained while reducing current, by power supplied from a normal electrical component driving battery (e.g., DC 12 V).
High voltage power sources such as described above generally typically include a booster circuit that is a DC-DC converter, and a capacitor capable of temporarily storing electric power. The booster circuit is relatively expensive and also is large in size, so even in engines where each of multiple cylinders are provided with injectors, high voltage is often supplied to each cylinder from a single set of booster circuit and capacitor. This arrangement is often used for engines around six cylinders or so in scale.
In such arrangements, the high-voltage source is usually designed with an optimized capacity of the booster circuit and capacitor, which repeatedly charges and discharges. The reason is that the high-voltage source is only temporarily used when opening the injector valves, and a design where the booster circuit could continuously apply large current to multiple cylinders at high voltage would result in an extremely expensive and large arrangement. For example, in a multi-cylinder engine with regular interval combustion (regular interval ignition), it is impossible for the injection start periods to overlap if injection is only performed once per cycle. Accordingly, the specifications for the high-voltage power source are decided by obtaining the minimum value for power source capabilities such that the above-described charge/discharge cycle will generally work at maximum revolutions.
However, there recently are cases that injection is divided into several times if the stroke period is long as compared to the fuel injection period, from the perspective of fuel mixture in the cylinder and formation of air-fuel mixture. For example, in the case of performing injection multiple times in an engine having three or more cylinders, so as to straddle the intake stroke and compression stroke, the injection start periods may overlap depending on the injection timing settings. Accordingly, there may be situations where the high-voltage power source substantially needs to be used at the same time. Examples of such a case include where a three-cylinder, 240° craft-shank angle (CA), regular-interval combustion engine has injection intervals of 240° CA for each of the multiple times of injection, and where a four-cylinder, 180° craft-shank angle (CA), regular-interval combustion engine has injection intervals of 180° CA for each of the multiple times of injection.
Such cases of using the high-voltage power source at multiple cylinders at the same time, or cases of using the high-voltage power source continuously in a short period of time, cause fluctuation in the current flowing to the injector and the applied voltage. The fluctuation results in a change in valve-opening properties and variance in fuel injection amount, thereby restricting injection timing settings. However, there are cases where a restricted timing is the optimal value for optimizing engine performance relating to fuel consumption, exhaust gasses, and so forth. This means that the engine performance cannot be optimized due to injector driving restrictions in such cases.
As examples of related art attempting to deal with this problem, Japanese Patent No. 3573001 describes grouping injectors with no overlapping operations, and providing capacitor to each group for storing electric power. Japanese Unexamined Patent Application Publication (JP-A) No. 2000-34589 describes disposing a switch element upstream of the capacitors, to select capacitors that have not discharged, so as to make a chargeable circuit configuration.
However, the art described in Japanese Patent No. 3573001 involves having to stop boosting while discharging, since using the booster circuit during discharging the capacitor may cause fluctuation in power source voltage. This is no problem regarding overlapping of single cylinders, but causes charging cycle interference among groups when performing charging operations multiple times in a short period of time.
On the other hand, selectively charging capacitors as in the art described in JP-A No. 2000-34589 enables the amount of time that the booster circuit can be used to be increased, allowing the restriction of charge/discharge cycle interference among groups to be eased to a certain degree, but not sufficiently.